Toughness enhancement and heterogeneous softening of a cryogenically cycled Zr–Cu–Ni–Al–Nb bulk metallic glass

In this study, as-cast and cold-rolled Zr_63.78Cu_14.72Ni₁₀Al₁₀Nb_1.5 bulk metallic glass (BMG) samples were subjected to 20, 70, 120, and 170 cryogenic thermal cycles prior to fracture toughness testing. Thermal cycling raised the fracture toughness by promoting plastic deformation and stable crack growth, with the most significant increase occurring after the first 20 thermal cycles. Thermally cycled samples showed more tortuous crack paths and stable crack propagation left periodic blunting marks spaced at ∼57.5 μm on the fracture surface corresponding to each increment of crack advance. Microhardness mapping revealed a microstructure of hard and soft domains (∼63 μm × 105 μm), and thermal cycling heterogeneously softened the hard domains while the soft domains remained apparently unchanged. In addition to the observed softening, large increases in the relaxation enthalpy, and thus the average free volume, were found over the first ∼70 thermal cycles. While cold rolling the samples prior to the thermal cycling did not raise the mean fracture toughness values, the scatter was reduced compared to as-cast thermal cycled samples. This was attributed to the introduction of shear bands giving a more repeatable initial microstructure than casting.